In recent years, there have appeared some notebook-type personal computers provided with a port for connecting a device conforming to the IEEE1394 standard. The IEEE1394 standard is for fast serial interfaces that enable data to be transferred at 100 Mbps or over. In 1995, the standard was officially adopted by the IEEE (Institute of Electrical and Electronics Engineers). The IEEE1394 standard supports three types of transfer rates at present; 100 Mbps, 200 Mbps, 400 Mbps and enables a total of 63 units to be connected. The maximum distance between those units is 4.5 m and the maximum distance between the outermost devices is 72 m. The IEEE1394 standard also enables a power supply between units (within a voltage range of 8 V and 40 V) through a bus conforming to the IEEE1394 standard (hereafter, referred to as an “IEEE1394 bus”). In addition, because the IEEE1394 standard enables units to be connected to each another without the use of a host computer such as a personal computer, etc., such an IEEE1394 bus is highly expected to be used as an infrastructure in each home network.
On the other hand, a notebook-type personal computer is generally provided with its own battery. With this built-in battery, for example, the user can use his/her notebook-type personal computer in such an environment as a train where no commercial power supply is available. Generally, such a built-in battery can be charged for repetitive use.
If a commercial power supply is available, the user can connect an AC adapter (a device for obtaining a DC output from a commercial AC input) to his/her notebook-type personal computer. Consequently, the battery built in the personal computer can be charged during operation.
Examples of the types of notebook-type PC batteries that can be charged include: a lithium-ion battery pack composed of two 4.2 V lithium-ion batteries connected serially (hereafter, referred to as a “two-serial battery pack”), a lithium-ion battery pack composed of three 4.2 V batteries (a “three-serial battery pack”), and a lithium-ion battery pack composed of four 4.2 V batteries (a “four-serial battery pack”) etc. Among these, the three-serial battery pack is most frequently used. This type of lithium-ion battery pack has an advantage in the aspect of power usage efficiency, which is higher than that for two-serial and four-serial battery packs. The three-serial lithium-ion battery pack has an output voltage range from about 9.0 V (at which the battery capacity becomes 0) to 12.6 V(=4.2 V×3).
FIG. 5 shows a notebook-type PC, provided with a three-serial lithium-ion battery pack, which is connected to a device conforming to the IEEE1394 standard via an IEEE1394 bus 150.
As shown in FIG. 5, the notebook-type PC composed as described above is further provided with an input circuit 142, a protection circuit 140, etc. The input circuit 142 includes a field-effect transistor (FET) provided on a power line L between a lithium-ion battery pack and an IEEE1394 bus 150 and used to switch connection/disconnection between a lithium-ion battery and an inner circuit 110 of the notebook-type PC. The protection circuit 140 is composed of two diodes D1 and D2.
A device 152A conforming to the IEEE1394 standard is provided with a power unit 154. On the power line from the power unit 154 to the IEEE1394 bus is provided a protection circuit composed of a diode D3. A device 152B is also composed just like the device 152A.
The protection circuits are provided on the power line of each of the notebook-type PC (diodes D1, D2) and devices 152A, 152B (diode D3) for the following reasons. Because the notebook-type PC and its circuits are connected to devices 152A, 152B via the IEEE1394 bus 150 in a cascade connection manner as shown in FIG. 5, electrical power is supplied to the IEEE1394 bus 150 from a power source which has the highest voltage among lithium-ion batteries and power units 154, provided in the notebook-type PC and the attached devices, respectively.
The power units 154 provided for both devices 152A and 152B can possibly output a voltage within the range 8 V to 40 V as allowed by the IEEE1394 standard. However, the inner circuit 110 of the notebook-type PC, as well as the inner circuits of both devices 152A and 152B may not withstand voltages at the higher end of this range and thus to prevent such voltages being applied from external sources, a diode D3 is connected to each power line in the manner shown in FIG. 5.
Furthermore, the two diodes D1 and D2 are provided in the notebook-type PC so that they compose the protection circuit 140 such that one of them can protect the inner circuit 110 of the PC when the other is damaged by a short-circuit caused by a fault, a trouble, etc. This is because another serious trouble (breakage, smoking, ignition, etc.) must be prevented for the reason of safety when any component is down. This is the reason why two diodes D1 and D2 are usually provided so as to protect the inner circuit against such troubles.
However, even when a protection circuit is composed of two diodes as described above, the conventional technique has been impossible to solve a problem that the use efficiency of the lithium-ion battery is low.
To take an example, in the case of the configuration shown in FIG. 5, the biggest drop caused by the input circuit 142 is usually about 0.3 V. The voltage drop of each of the diodes D1 and D2 is typically about 0.6 V, so the biggest total voltage drop in the power line L is about 1.5 V. In order to supply a voltage of 8.0 V to a device conforming to the IEEE1394 standard, therefore, the voltage of a lithium-ion battery cannot go outside a range of about 9.5 V(=8.0 V+about 1.5 V) to 12.6 V. Consequently, this configuration does not permit use of the full capacity of the lithium-ion battery. Therefore, the power supply to the object device corresponding to the IEEE1394 standard must be stopped even when the residual capacity is still enough.
Generally, a notebook-type PC requires a voltage of 8 V or under (ex., 7.5 V). For a battery that does not conform to the IEEE1394 standard, therefore, the notebook-type PC can lower the output voltage up to the above requirement. For a battery conforming to the IEEE1394 standard, however, the conventional technique, which causes a large voltage drop in the power line, sometimes disables supply of electrical power to external devices if the voltage of the battery drops. To avoid such a problem, therefore, the notebook-type PC is provided with an internal boosting DC/DC converter, so as to boost the output voltage to 8 V or over. This causes an increase in the manufacturing cost of the notebook PC.
Under such circumstances, it is an object of the present invention to provide a power unit that can ameliorate the aforementioned problems and improve the usage efficiency of the battery connected to a power line, as well as a computer that can improve the usage efficiency of the battery built therein.